The present invention relates generally to the measurement of transmitter parameters, and more particularly to a method and apparatus for in-service measurement of a transmitter's magnitude and phase nonlinearities.
Nonlinear magnitude and phase responses are common distortions caused by a transmitter power amplifier that affects the quality of a transmitted signal. The gain and phase responses of the amplifier are a function of the input signal magnitude to the amplifier, which can drive the amplifier into nonlinear operation. The distortion present in a received signal from the transmitter is often a mixture of linear distortion, nonlinear distortion, phase noise and additive noise. Correctly separating the nonlinear errors from the other distortions is desired for accurate measurement. However, with in-service measurement, the received signal is data bearing and its content is unknown to the measurement instrument.
In an 8 level Vestigial Side Band (8-VSB) digital modulation system, for example, the overall system response of the combined transmitter and receiver corresponds to a raised cosine filter to avoid system generated intersymbol interference. The system response is implemented with nominally identical root raised cosine filters in the transmitter and in the receiver. The information bearing digital data stream is randomized for spectrum spreading over the bandwidth of the frequency channel. The randomized data is forward-error-corrected (FEC) coded and interleaved. The data is trellis encoded as an 8-level (3-bit) one dimensional constellation with the outputs of the trellis coder referred to as symbols that are one of eight symmetric odd-valued integer levels from -7 to +7 units. To aid synchronization in low signal to noise and/or high multipath situations, segment and field syncs are inserted in the 10.76 Msymbols/sec symbol stream as well as a small pilot tone at the carrier frequency generated by offsetting the real or I channel of the complex signal containing the data and the sync pulses by 1.25 units. At the transmitter, the composite signal passes through a root raised cosine filter and modulates an intermediate frequency carrier signal which is up-converted to an RF frequency for transmission at the desired channel frequency. The offset causes the pilot tone to be in-phase with the I channel carrier frequency. Alternately, the composite signal may directly modulate the RF carrier.
Synchronous demodulation may be used to detect the eight constellation decision levels. However, the constellation of the transmitted symbols may bend or stretch to form certain curvatures due to the nonlinear distortions in the transmitter. Therefore, for estimation of constellation decision levels, a conventional slicing method is inadequate for symbol decision in the presence of large nonlinear distortions. For measurement purposes, attempting to determine an ideally transmitted signal from the received signal with large transmitter phase and/or magnitude nonlinearity would not be reliable using the conventional slicing method.
Also, conventional methods of measuring transmitter nonlinearity that apply the band-limiting transmission system's receiver filter to the received intermediate frequency (IF) signal would alter the characteristics of the nonlinear distortions. Nonlinear and linear transfer functions are usually not commutative, as stated in FIG. 1. That means the nonlinear function observed from the demodulated baseband signal (with the use of the transmission system's receiver filter) is different from the original nonlinear function of the power amplifier. It is difficult to derive the original nonlinear function from what is observed from the demodulated baseband signal, especially with a randomized digital signal. Also, strong nonlinearity causes signal spectrum spreading. The transmission system's receiver filter will significantly attenuate the out-of-band portion of the spread spectrum signal with the loss of spectral information characterized by the nonlinear distortions.
To determine the causes of poor signal quality of the transmitted signal and to provide pre-correction characteristics for the transmitter, accurate measurement of transmitter nonlinearities is very useful. However, the transmitter nonlinearities in the received signal are often combined with other distortions, such as linear distortions, carrier phase jitter, and additive noise. Due to the difficulty of separating various errors, nonlinearity is traditionally measured in an out-of-service mode. An out-of-service mode requires turning off the broadcast channel(s) (i.e. removing the modulations), which causes the loss of nonlinearity characteristics caused by the presence of the digital signal as well as other distortions.
What is needed is a method and apparatus for in-service measurement of transmitter magnitude and phase nonlinearities of a received signal, where the signal has a mixture of linear distortions, nonlinear distortions, phase noise and additive noise present, that uses reliable estimation of constellation decision levels and preserves original spectral information.